The invention relates to an inking unit for a printing machine, which includes one or more distributor drums, ink transfer rollers, ink application rollers, an ink reservoir for printing ink and an ink metering device having at least one electrically controllable valve for metering the printing ink. The invention also relates to a method for supplying printing ink to a printing machine.
A conventional inking unit for an offset printing press is shown in FIG. 7. The inking unit includes an ink fountain 122 serving as a reservoir for printing ink 124, an ink fountain roller 126, an ink feed roller 128 and a roller arrangement which, in this conventional embodiment, includes four distributor drums 130a to 130d, four ink applicator rollers 132, which roll on a plate cylinder 134 of the printing machine, and a number of ink transfer rollers 136. The demand or requirement for the printing ink 124 is set or adjusted zonewise as a function of or in accordance with the printing image by pressing a sprung blade (ink fountain blade) 125 more-or-less strongly against the ink fountain roller 126, and thereby varying the ink quantity that passes through the gap between the ink fountain blade 125 and the ink fountain roller 126. Regulation of the printing ink 124 over the entire width of the ink fountain roller 126 is performed by varying the ink removing stripe that is lifted from the ink fountain roller 126 by the ink feed roller 128 in a rhythmic oscillating movement. The printing ink 124 is repeatedly distributed and divided by the downline arrangement of hard and soft rollers of various diameters, and distributed by an axial movement of the distributor drums 130a to 130d before it is applied to the plate cylinder 134 by the ink applicator rollers 132.
The aforedescribed arrangement permits only relatively imprecise ink metering which is yet further worsened by wear in the ink metering region, with the result that a large number of rollers are required to achieve the necessary low ink film thickness. The ink feed can be reproduced only with difficulty. Subassemblies such as that formed of an ink fountain, an ink fountain roller and ink feed roller require a relatively large amount of space and have to be cleaned regularly. In order to automate the ink metering, which is normally performed manually, electric actuating and control elements are required in addition to the elements described hereinabove. There is also a need for a squeegee device to remove excessive printing ink. Heretofore known methods for solving these problems call for the use of ink cartridges, for example, as a result of which only the manual ink feed is eliminated, or for the use of low-wear materials for the ink metering region, which likewise only partially solves the described problems.
The journal Zeitungstechnik, July/August 1996, page 30, displays an inking unit of the type described in the introduction hereto, wherein the printing ink is applied to a specifically constructed ink film roller with the aid of a digital ink pumping system which includes a gear pump and solenoid valves which are under pulse-length control. It is true that the ink metering is performed more precisely than with ink fountain systems, but there is a need for a squeegee or doctor blade device for removing excessive ink.
It is accordingly an object of the invention to provide an inking unit for a printing machine with an ink metering device that permits extremely precise ink metering, the inking unit having a simple and space-saving construction and being derivable from a conventional inking unit by simple structural changes. It is a further object of the invention to provide a method of supplying ink to a printing machine that is an improvement over heretofore known methods of this general type.
With the foregoing and other objects in view, there is provided, in accordance with one aspect of the invention, an inking unit for a printing machine, including at least one distributor drum, ink transfer rollers, ink application rollers, an ink reservoir for printing ink and an ink metering device having at least one electrically controllable valve for metering the printing ink, comprising a high-pressure pumping device for supplying the printing ink metering device with printing ink at a predetermined high pressure from the ink reservoir, and a heating device for heating the printing ink to a predetermined temperature above room temperature, the at least one valve of the ink metering device having a short reaction time and a flow rate with a low dependence upon viscosity.
In accordance with another feature of the invention, the at least one valve includes piezoelectric actuators.
In accordance with a further feature of the invention, the at least one valve includes a spherical sealing element disposed opposite a spherically recessed valve seat wherein an opening having a substantially smaller diameter than that of the spherical sealing element is formed.
In accordance with an added feature of the invention, the printing ink, at room temperature and a low shear rate, has a dynamic viscosity of more than 10 pa s (pascal seconds), the pumping device being capable of producing a pressure in the range of approximately 10 to 100 bar, and the temperature of the printing ink heated by the heating device being in a range of approximately 40xc2x0 C. to 70xc2x0 C.
In accordance with an additional feature of the invention, the ink metering device includes a row of nozzles directed onto the surface of one of the distributor drums.
In accordance with yet another feature of the invention, a respective valve is provided for each ink zone, the valve having an outlet connected to at least one nozzle disposed in the respective ink zone.
In accordance with yet a further feature of the invention, the ink reservoir is one of an ink cartridge and a bulk container, respectively.
In accordance with yet an added feature of the invention, the inking unit includes a valve for supplying cleaning liquid for the purpose of cleaning the ink metering device.
In accordance with yet an additional feature of the invention, the ink metering device is provided for the purpose of spraying cleaning liquid onto a distributor drum.
In accordance with another aspect of the invention, there is provided a method for supplying printing ink to a printing machine, the printing ink being metered by at least one electrically controllable valve, which comprises feeding the printing ink to the at least one valve at a predetermined high pressure and at a predetermined temperature above room temperature, the printing ink being metered free of excess in the printing machine in accordance with the demand therefor.
In accordance with another mode, the method of the invention includes providing a printing ink which, at room temperature and a low shear rate, has a dynamic viscosity of more than 10 Pa s, pressurizing the printing ink in the range of approximately 10 to 100 bar, and heating the printing ink to a temperature in the range of approximately 40xc2x0 to 70xc2x0 C.
In accordance with a further mode of the method, the at least one valve has a reaction time in the order of magnitude of microseconds, and the method further includes metering the ink by opening and closing the at least one valve under pulse-length control.
In accordance with a concomitant mode, the method of the invention includes spraying the printing ink out of synchronism with the machine time cycle onto a distributor drum of the printing machine so that, with one of each distributor stroke and each distributor revolution, respectively, the printing ink is applied offset temporally relative to the machine time cycle.
Thus, the inking unit according to the invention permits the use of a standard offset printing ink which, at room temperature and a low shear rate, has a dynamic viscosity of more than 10 Pa s, for example 50 to 70 Pa s. By virtue of the fact that the printing ink is fed to the at least one valve at a pressure in the range of approximately 10 to 100 bar, and is heated in the process to a temperature in the range of approximately 40 to 70xc2x0 C., the viscosity is reduced so far that highly precise ink metering is possible in conjunction with a satisfactorily short reaction time of the valve. By selecting a suitable valve construction, assurance is provided that the flow rate of the valve depends as little as possible upon the viscosity, with the result that the desired ink dosage is virtually uninfluenced by any inhomogeneities in the printing ink or by temperature fluctuations.
The at least one valve is preferably a valve with piezoelectric actuators as adjusting elements, a so-called piezoelectric valve. As described in the published German Patent Document DE-A-4 220 177, for example, such a valve has reaction times of the order of magnitude of microseconds. Consequently, it can be opened and closed many times more quickly than conventional solenoid valves, which have reaction times of several milliseconds. This permits very fine metering of the printing ink, preferably by complete opening and closing of the valve under pulse-length control, or alternatively by regulating the flow rate of a valve, the extent of opening of which is controlled by a fast control loop.
Ink metering by piezoelectric actuators has become known heretofore per se in the technology of inkjet printers. Inkjet printers spray ink of low viscosity, normally directly onto paper, and in one of the conventional techniques, the ink is metered by compressing a small volume of printing ink briefly by a piezoelectric actuator, a droplet of printing ink being sprayed onto a dot. By contrast therewith, in the case of the invention of the instant application, it is possible to use offset printing ink of relatively high viscosity, and the compressed and heated printing ink is already under a sufficiently high pressure for ink spraying, with the result that it need only be metered by the piezoelectric valve.
A preferred valve construction for ink metering which is as independent of viscosity as possible is a valve with a spherical sealing element disposed opposite a spherically recessed valve seat wherein an opening with a substantially smaller diameter than that of the spherical sealing element is formed. The volumetric flow is determined by a nozzle connected downline of the valve. In this regard, this nozzle can have a very short through-flow channel, as a result of which the mode of operation corresponds to that of an orifice.
In a preferred embodiment, the ink metering device includes a row of nozzles which are directed onto the surface of one of the distributor drums. The distributor drum moves reciprocatingly in the machine time cycle along the longitudinal axis thereof, so that the printing ink is distributed laterally. In the case of opening and closing of the at least one valve under pulse-length control, care must be taken that the ink spraying not be performed synchronously with the machine time cycle and the distributor movement, respectively, which would produce an inhomogeneous ink profile. The ink film that is applied can be homogenized, for example, by providing that the instant of ink application be somewhat offset temporally with each distributor stroke or with each distributor revolution, or for example by temporal modulation of the valve control pulses.
The invention is suitable both for conventional offset printing presses (sheet-fed and web-fed machines) with ink zones, and for inking units without zones. In one embodiment for ink zones, there is provided for each ink zone a valve having an outlet that is connected to one or more nozzles which are situated in this ink zone. The more nozzles that are present per ink zone, the more uniform the thickness of the ink film that is applied becomes. If spraying onto a distributor drum is performed with variable control times, as described hereinabove, a sufficiently uniform ink distribution can be achieved with a single nozzle per ink zone. On the other hand, in the case of the use of many nozzles side by side, it is also possible to spray onto a quickly running roller other than a distributor drum.
The ink reservoir can, for example, be an ink cartridge which is located in a pressure vessel and is connected to the at least one valve via a pressure line. The ink is pressurized by pressurizing the interior of the pressure vessel. The required pressure can easily be produced from the compressed air present in a printing plant by a pressure converter. An alternative to supplying ink by cartridges is a supply from a bulk container, for example, supplying ink in a manner well known in the technology via a barrel, in which case no change of cartridge is necessary.
As mentioned, the inking unit according to the invention permits ink metering in a manner that is so precise and uniform that considerably fewer inking rollers suffice than were previously required. Moreover, conventional subassemblies such as ink fountain and fountain roller are dispensed with. No special components are required for the remaining part of the inking unit, but conventional components, in particular conventional inking rollers, can be used. As a result, a conventional inking unit can easily be converted to an inking unit according to the invention.
The highly precise ink metering in accordance with the invention permits the printing ink to be metered in the printing machine in accordance with its requirements free from excess, so that it is possible to dispense with any sort of squeegee or doctor blade device. If, nevertheless, an operation is performed with a given ink excess and a squeegee device is consequently used, the quantity of ink to be removed by squeegee can be considerably smaller.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as an inking unit for a printing machine and a method of supplying ink to a printing machine, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, wherein: